Biofuels for transportation are attractive replacements for gasoline and are rapidly penetrating fuel markets as low concentration blends. Biofuels, derived from natural plant sources, are more environmentally sustainable than those derived from fossil resources (such as gasoline), their use allowing a reduction in the levels of so-called fossil carbon dioxide (CO2) gas that is released into the atmosphere as a result of fuel combustion. In addition, biofuels can be produced locally in many geographies, and can act to reduce dependence on imported fossil energy resources. Two alcohols useful in biofuels are ethanol and butanol.
Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2002 was an estimated 10.8 billion gallons. The global market for the fuel ethanol industry has also been predicted to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA and several developing nations.
Butanol may be used as a fuel in an internal combustion engine. It is in several ways more similar to gasoline than it is to ethanol. As the interest in the production and application of environmentally sustainable fuels has strengthened, interest in biological processes to produce butanol (often referred to as bio-butanol) has increased. For example, in June 2006 BP announced collaboration with Dupont and British Sugar to manufacture biobutanol using conventional technology in the UK. BP provides a route for butanol into the transport fuel market and has stated that it aims to blend butanol with petrol at its 1200 filling stations.
Butanol may be produced by microbial fermentation of biomass from crops such as sugar beet, corn, wheat and sugarcane. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed and the cultivation of starch or sucrose-producing crops for butanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel butanol.
Crude glycerol is a by-product or waste-product produced in many industries. For example, waste produced during biodiesel production includes crude glycerol. Glycerol is also produced during production of palm kernel oil methyl esters, in processes involving fat saponification, alcoholic beverages manufacture, and processes used in the oleochemicals industry. The increased use of such processes worldwide is resulting in a surplus of glycerol which must be disposed of, typically in land fills. Disposal poses an environmental risk and can be costly. Purified glycerol has many commercial uses but its recovery from industrial waste can be expensive.
A number of industrial processes produce acetate as a by-product. For example, production of ethanol by microbial fermentation is always associated with co-production of acetate and/or acetic acid. Unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. In addition, acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to Green House Gas emissions.
CO is a major by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually. The release of CO into the atmosphere may have significant environmental impact. In addition, emissions taxes may be required to be paid, increasing costs to industrial plants.
Butanol can be produced by microbial fermentation of glycerol with or without acetate as a co-substrate. By way of example, Heyndrickx et al (Appl Microbiol Biotechnol (1991) 34; 637-642) report fermentation of commercial grade glycerol alone or in combination with acetate to various products including butanol and/or ethanol using Clostridium butyricum and Clostridium pasteurianum. However, such methods may have low efficiency. In addition, Hendrickx et al report that in the case of Clostridium pasteurianum there is substantially no net acetate consumption during fermentation of glycerol and acetate to butanol.
Bibliographic details of the publications referred to herein are collected at the end of the description.